In recent years, with increased concerns about environmental problems, much research has been carried out into electric vehicles (EV) and hybrid electric vehicles (HEV), which are capable of substituting for vehicles using fossil fuel, such as gasoline and diesel oil, which cause air pollution. Nickel-metal hydride (Ni-MH) secondary batteries have been mainly used as a power source for the electric vehicles and the hybrid electric vehicles. On the other hand, much research has also been carried out into lithium secondary batteries having high energy density, high discharge voltage, and high output stability, and some of the lithium secondary batteries are now commercialized.
In particular, it is necessary for lithium secondary batteries used for electric vehicles to exhibit high energy density and to provide high output within a short time. In addition, it is also necessary for lithium secondary batteries used for electric vehicles to be used for 10 years or more under severe conditions. For these reasons, lithium secondary batteries used for electric vehicles require higher safety and longer lifespan than conventional small-sized lithium secondary batteries. In addition, secondary batteries used for electric vehicles (EV) and hybrid electric vehicles (HEV) require excellent rate characteristics and excellent power characteristics based on the operation conditions of a vehicle.
Carbon materials are mainly used for negative electrode active materials of a lithium secondary battery, and the use of lithium metals and sulfur compounds is under consideration. In addition, lithium-containing cobalt oxides, such as LiCoO2, are mainly used for positive electrode active materials of a lithium secondary battery, and the use of lithium-containing manganese oxides, such as LiMnO2, having a layered crystal structure, LiMn2O4, having a spinel crystal structure, and lithium-containing nickel oxides, such as LiNiO2, is also under consideration.
Among the positive electrode active materials, LiCoO2 is widely used due to its excellent lifetime characteristics and high charge and discharge efficiency, but has low structural stability. In addition, due to the resource limitations of cobalt, which is a raw material, LiCoO2 is expensive, and therefore price competitiveness is low, whereby the massive use thereof as power sources in fields such as electric vehicles is limited.
LiNiO2 is relatively inexpensive, and makes it possible for a battery to have high discharge capacity. However, phase transition abruptly occurs in the crystal structure of LiNiO2 depending on the change in volume of the battery caused by charge and discharge of the battery. In addition, when LiNiO2 is exposed to air and moisture, the safety of LiNiO2 is abruptly lowered.
In addition, lithium manganese oxides, such as LiMnO2 and LiMn2O4, exhibit excellent thermal safety and are inexpensive. However, the use of lithium manganese oxides entails a small charge capacity, poor cycle characteristics, and poor high-temperature characteristics.
For these reasons, much research has been carried out into new positive electrode active materials that have structures other than the above-mentioned structures.
For example, research has been carried out into an oxide containing excessive lithium, e.g. a lithium transition metal oxide containing a high content of Mn, wherein the content of lithium is higher than the content of the transition metal such that a high capacity of 270 or more mAh/g is exhibited at a high voltage of 4.5 V or higher.
However, the oxide containing excessive lithium has a high irreversible capacity. Furthermore, in addition to lithium, oxygen escapes from the active material structure at the time of high-voltage activation to utilize excess lithium. As a result, the active material structure may collapse, and a voltage sagging phenomenon may occur, whereby the deterioration of the battery cell may be accelerated.
Meanwhile, many researchers have proposed methods of using a positive electrode active material containing Li2MnO3 in a layered structure in order to secure the structural stability of the positive electrode active material having such a layered structure. In this case, the positive electrode active material contains a large amount of Mn. As a result, the positive electrode active material is very cheap and exhibits large capacity and high stability at high voltage. After the activation of a flat range of 4.4 V to 4.6 V, however, the transition from the layered structure to the spinel structure is performed, and therefore the contact between domains is weakened. As a result, the positive electrode active material may be excessively structurally changed, whereby the improvement of electrical properties may not be satisfied.
That is, the structure of a secondary battery that exhibits desired lifetime characteristics and safety has not yet been proposed.